A Dictionary of Modern Written Arabic
Wehr, Hans, and J. Milton Cowan. A Dictionary of Modern Written Arabic. London: Harrap [etc.], 1976.
September 22, 2012
Most comprehensive Arabic dictionary
This has been the most complete Arabic-English dictionary produced for decades. While some modern dictionaries are easier to use and may be appropriate for casual translation, Hans Wehr's dictionary is more thorough and accurate for Modern Standard Arabic. The translations include not just the common denotations of meaning but also the more rarely understood connotations. If you want to understand the nuances of a word in Arabic, read through the whole definition, across the root and all the forms of the word. No other Arabic-English dictionary comes close to this level of detail.
This dictionary does not contain any colloquial words and may be missing some newer uses of standard words, so it's not the ONLY dictionary you'll ever need. But it IS the first Arabic dictionary you should have - I've had three copies over 3 decades and am thrilled to see it here in electronic form for any student of Arabic to use.